The fuel cost increase and introduction of environmental regulations have increased the demand for improving fuel economy of automobiles in recent years. Rubber compositions for producing a tread, which occupies a larger portion of a tire among various tire components, have been required to be excellent in fuel economy.
Known as a method for improvement in fuel economy is a method in which the amount of a filler such as carbon black is decreased. However, the decrease in the amount of a filler tends to reduce rubber strength and deteriorate abrasion resistance; therefore, it has been difficult to achieve both fuel economy and abrasion resistance at high levels. In the rubber compositions, degradation resistance has been demanded as well as fuel economy and abrasion resistance. A method has thus been required in which both good fuel economy and abrasion resistance are achieved while good degradation resistance is maintained.
Patent Documents 1 to 3 propose reducing rolling resistance with modified diene rubbers such as modified butadiene rubber and modified styrene butadiene rubber. Patent Documents 4 and 5 propose improving rubber strength with a natural rubber in which proteins have been removed. However, there is still room for improvement in that both fuel economy and abrasion resistance need to be achieved at higher levels while good degradation resistance is maintained.
In respect of tires (particularly heavy-load tires), a method in which natural rubber or butadiene rubber is used as a rubber component is common for improvement in fuel economy. Known as a method for further improvement in fuel economy are a method in which silica is used as a filler and a method in which the amount of a filler is decreased. However, these methods may reduce rubber strength and fail to maintain good abrasion resistance. In particular, rubber compositions for heavy-load tires need high rubber strength and it is therefore difficult to apply the methods to these rubber compositions. In addition, conventional silane coupling agents used in combination with silica tend, for example, to increase viscosity upon mixing and to reduce scorch time, and thus may cause any adverse effects on processability. Accordingly, a method that achieves well-balanced improvement in fuel economy, abrasion resistance, and processability is required.
Patent Document 6 discloses a rubber composition prepared using natural rubber and epoxidized natural rubber in order to increase the content of non-petroleum resources. However, the rubber composition still has room for well-balanced improvement in fuel economy, abrasion resistance, and processability.
Patent Document 1: JP 2001-114939 A
Patent Document 2: JP 2005-126604 A
Patent Document 3: JP 2005-325206 A
Patent Document 4: JP H08-12814 A
Patent Document 5: JP H11-12306 A
Patent Document 6: JP 2007-169431 A